In a variety of applications, implantable medical devices are used for one or both of monitoring or delivering therapy to a patient. For example, cardiac pacemakers typically monitor electrical signals from the heart, i.e., an electrocardiogram (ECG), and deliver electrical stimulation to the heart, via electrodes. The electrodes may be positioned within the heart, and coupled to the pacemaker by intravenous leads, or may be positioned subcutaneously using any non-intravenous location, such as below the muscle layer or within the thoracic cavity, for example.
In the case of demand pacing, for example, a cardiac pacemaker monitors the ECG to determine whether an intrinsic cardiac depolarization, e.g., a P-wave or R wave, occurs within a rate interval. If an intrinsic depolarization occurs, the pacemaker resets a timer and continues to monitor the electrical signals from the heart. If an intrinsic depolarization does not occur, the pacemaker delivers one or more electrical pulses to the heart, and resets the timer.
As another example, an implantable medical device may monitor electrical signals within the brain, e.g., an electroencephalogram (EEG), sensed via electrodes. An implantable medical device may monitor the EEG to, for example, identify epileptic seizures, or other neurological issues. In some cases an implantable medical device may deliver electrical stimulation to the brain, or other tissue within the patient, in response to or based on the analysis of the EEG. In other examples, implantable medical devices may monitor any of a variety of signals generated by any of a variety of sensors based on physiological parameters of a patient, such as pressure, impedance, temperature, or physical motion.
Historically, implantable medical devices have used analog circuitry to process or analyze such physiological signals. For example, many pacemakers have used analog circuitry to process the ECG, e.g., to detect P-waves and R-waves. More recently, use of digital signal processing for this purpose has been considered or implemented.
Digital signal processing requires conversion of the analog signal, e.g., ECG, to a digital signal using an analog-to-digital converter (ADC). One type of ADC is a delta-sigma ADC. A delta-sigma ADC tracks the changes in the analog input signal by comparing the input signal to a feedback signal.
In general, based on its complexity and sampling rate, there is a limit to the magnitude and rate of change of an input signal that a delta-sigma ADC can track. Implantable medical devices can be exposed to electro-magnetic interference (EMI) that can induce large voltages on the leads or sensor inputs. In the presence of EMI, a delta-sigma ADC can be overloaded so that incorrect data streams are generated, which may be incorrectly interpreted by a system that analyzes the output digital signal. For example, a cardiac pacemaker may incorrectly interpret low frequency shifts in the output digital signal caused by EMI to be intrinsic cardiac activity. In this case, a pacemaker operating in a demand mode may incorrectly inhibit delivery of pacing pulses in the presence of EMI.